1. Field of the Invention
This invention relates generally to a directional drilling system run on coiled tubing, and particularly to a system where the bent housing of the drilling motor is oriented by an associated electric motor relative to the coiled tubing in a manner such that the trajectory of the borehole is steered.
2. Description of the Related Art
A directional or deviated borehole typically is drilled by using a downhole motor, a bent housing, and a bit that are suspended on drill pipe that extends upward to the surface. The drill pipe can be rotated at the surface to orient the bent housing in order to control the tool face angle and thus the azimuth at which the borehole is drilled. The motor is powered by pumping a weighted drilling mud down the drill string and through the motor, and the mud has sufficient hydrostatic pressure to prevent any hydrocarbons from entering the borehole and creating hazardous and dangerous conditions at the surface. However, it is believed that the high hydrostatic pressure tends to impede the progress of the drilling by holding the chips or particles of rock that are loosened by the bit down on the bottom of the borehole so that the cleaning action of the mud as it emanates from the bit nozzles is not as efficient as desired.
A work string that can be run into a wellbore that is under pressure is coiled tubing, which is a long, jointless metal conduit that is wound on a large diameter reel at the surface. The reel, pumps and guides are mounted on a mobile surface unit, and an injector is used to drive the tubing into and out of the well under pressure through blowout preventers. Although this type of tubing has been used extensively for workover operations such as sand clean out, it cannot be rotated at the surface to achieve directional steering of a drilling motor and bent housing. However, this system is well suited for balanced or slightly underbalanced drilling to reduce or eliminate chip hold-down, and thereby permit a faster rate of penetration of the bit.
Another desirable feature in directional drilling with a downhole motor and bent housing is the ability to rotate the housing continuously so that its bend point merely orbits around the borehole axis so that the bit can drill straight ahead, rather than along a curved path. The ability to drill both curved and straight borehole sections enhances the drilling toward a particular target in the earth. When the drilling tools are run on drill pipe, this is readily accomplished by superimposing rotation of the drill pipe over that of the motor output shaft. However, when the same system is run on coiled tubing, this cannot be done. The present invention is uniquely arranged with a downhole electric motor that is employed to orient the bent housing relative to the lower end of the coiled tubing to achieve a selected tool face angle, or to continuously rotate the bent housing when desired for straight ahead drilling. The electric orienting motor is powered by an electric cable that extends to the surface through the coiled tubing.
This same electric cable also can be used to telemeter numerous borehole, motor performance and formation characteristic measurements uphole. The drilling process can be automatically controlled from the surface, and the angular orientation of the bent housing set at any desired value.
An object of the present invention is to provide a new and improved directional drilling system that is run on coiled tubing and used to drill a well that is under pressure.
Another object of the present invention is to provide a new and improved directional drilling system of the type described which includes an electric motor to orient the bent housing to achieve a selected tool face during curved-hole drilling, or to continuously rotate the bent housing to achieve straight-ahead drilling.
Still another object of the present invention is to provide a new and improved directional drilling system of the type described which includes means for measuring and transmitting to the surface various borehole, formation and drilling tool properties and characteristics that are useful in evaluation and automatic control of the drilling.